1. Field of the Invention
The present invention relates to a data recording/reproducing apparatus and a method thereof, in particular, to a data recording/reproducing apparatus for securing the continuity of data and a method thereof.
2. Description of the Related Art
In recent years, apparatuses that record and reproduce picture/sound data to and from a randomly accessible record/reproduction medium instead of a sequentially accessible record/reproduction medium such as a tape are becoming common.
In a picture recording/reproducing apparatus that records and reproduces picture data in a high quality, the data transmission rate should be high and a large storage capacity is required to record picture data for a ling time. Thus, a plurality of hard disks are operated in parallel for a high transmission rate. In addition, parity data is recorded so that even if one of hard disks gets defective, original data can be restored (re-structured). Thus, the reliability of the picture recording/reproducing apparatus is improved.
Such a picture recording/reproducing apparatus is referred to as a disk array unit. Such a disk array unit is described in "A Case for Redundant Arrays of Inexpensive Disks (RAID)," ACM SIGMOD, Conference, Chicago, Ill., June, 1988. Referring to the paper, disk array units are referred to as RAIDs. RAIDs are categorized as RAID 1 to RAID 5. FIG. 1 shows a disk array unit corresponding to RAID 3. Referring to FIG. 1, data with a predetermined bit width such as eight bits or 16 bits is supplied to a disk array controller 2. A plurality of hard disks 3, 4, 5, 6, and 7 (in this example, five hard disks) are connected to the disk array controller 2. The hard disks 3, 4, 5, and 6 are used to record data. The disk array controller 2 assign data to the hard disks 3, 4, 5, and 6. On the other hand, the hard disk 7 is used to record only parity data calculated corresponding to input data.
FIG. 2 is a schematic diagram for explaining a generating operation of parity data for example odd parity data. Four data D1, D2, D3, and D4 are supplied to an exclusive OR gate 36. The exclusive OR gate 36 generates parity data DP. In other words, the parity data DP is generated in such a manner that the number of "1s" of each bit of the data D1, D2, D3, and D4 is an odd number. The data D1 to D4 are data recorded on the hard disks 3, 4, 5, and 6, respectively. The parity data DP is recorded on the hard disk 7 of the disk array unit shown in FIG. 1.
If any hard disk gets defective, by calculating the remaining data and the parity data DP, the data recorded on the defective hard disk can be restored. Thus, when the restored data is re-recorded on a substitute one of the defective hard disk, data can be re-structured. Normally, the defective hard disk is substituted with a new one. For example, as shown in FIG. 3, if the data D4 (recorded on a hard disk 3) of the data D1 to D4 becomes error data, when the data D3 is detected as error data, by inputting the normal data D1, D2, and D4 and the party data DP to the exclusive OR gate 36, the original data D3 can be restored. When the hard disks 3, 4, 5, 6, and 7 perform an error correction operation with error correction code and an uncorrectable error takes place, the error is corrected with the above-described parity data.
In the case that such a picture/sound data recording apparatus is used for a broadcast operation, when an input picture data contains a D2 base band signal, a transmission rate of around 120 Mbps is required. Assuming that the effective transmission rate of each hard disk is around 30 Mbps, four hard disks are required in each disk array unit. For example, when six channels are required, 24 hard disks are required.
The effective transmission rate is obtained by dividing an amount of data by a total time period necessary for reading/writing the data. Thus, the effective transmission rate is not the transmission rate of each hard disk or an instantaneous transmission rate of an interface portion. In addition, it is assumed that a plurality of pictures or the same picture is reproduced or recorded from/to the same recording/reproducing apparatus through individual channels.
However, it is not practical to operate as many as 24 hard disks in parallel at a time. In other words, when many hard disks are operated at a time, the management side that issues commands and manages these hard disks is excessively loaded. Thus, it takes a time for the management side to control the hard disks. Consequently, the effective transmission rate decreases, thereby deteriorating the reliability against a defect. Conventionally, to solve such a problem, a plurality of disk array units are operated in parallel and thereby the transmission rate is increased.
However, in a picture recording/reproducing apparatus with conventional disk array units, even if one of hard disks gets defective and thereby data should be re-structured, when a required amount of data is not supplied to the user (namely, when an operation that causes the transmission rate to extremely decreases is performed), the data stops and thereby the reproduction of picture data and sound data stops. From this point of view, data cannot be freely re-structure.
On the other hand, in the case that data can be re-structured by the user at a sacrifice of the performance, until the re-structuring operation is performed, the apparatus is operated without redundancy.
Generally, a hard disk uses a SCSI interface. With the SCSI interface, when a command for reproducing or recoding data from or to a hard disk is sent to a hard disk, it takes a time to send a new command for stopping the current operation or performing the next operation. In other words, in the system of which the disk array unit automatically starts the re-structuring operation and the disk array unit forcedly stops the re-structuring operation and reproduces or records the requested data when it receives a command from the user, it takes a long time for the disk array unit to start an operation after receiving a command. In this method, the continuity of data cannot be secured. Consequently, moving picture data or sound data may not be continuously reproduced.
In a system that predicts a delay for a real operation, to continuously reproduce moving picture data and sound data, a memory for storing extra data and a controlling circuit therefor are required, thereby raising the cost.
When a data (such as picture data and/or sound data) recording/reproducing apparatus has a disk array unit that re-structures data for securing the reliability, if part of data should be re-structured due to a defect of a hard disk, the redundancy cannot be restored without a sacrifice of part of the function.